Serum thymic factor (facteur thymique serique, FTS) is a thymic hormone that induces in vitro differentiation of T-cell precursors into more mature cells with T-cell characteristics (Incefy et al., Clin. Exp. Immunol. 40, 396 (1980), Bach et al. Bull. Inst. Pasteur 76, 325 (1978)). FTS is reported to be present in the thymus (Monier et al., Clin. Exp. Immunol. 42, 470 (1980)) and its presence in serum requires the presence of the thymus (Dardenne et al., Immunology 27, 299 (1974)). As isolated from porcine serum, FTS is a nonapeptide of the amino acid sequence Glp-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn.
The amino terminus is blocked by a pyroglutamic acid (Glp) residue (Bach et al., Nature, 226, 55 (1977); Dardenne et al., J. Biol. Chem. 252, 8040 (1977), Pleau et al., J. Biol. Chem. 252, 8045 (1977). FTS in human serum occurs as the zinc complex, thymulin (Dardenne et al., C.R. Acad. Sci. (Paris) 292, 793 (1981)).
FTS activity is detectable in the circulating blood of man and animals by a bioassy called the rosette inhibition assay (Incefy et al., Clin. Exp. Immunol. 40, 396 (1980); Iwata et al., Am. J. Med. 71, 383 (1981). The amount of FTS activity detected by this assay generally reflects thymic development and function. Other thymic hormones, such as thymopoietin (Iwata et al., Biochem. Biophys. Res. Comm. 88, 1419 (1979) and thymosin-.alpha..sub.1 (Wong and Merrifield, Biochemistry 19, 3238 (1980) and certain of its segments (Ciardelli, et al.
Biochemistry 21, 4233 (1982)) as well as an allogenic factor from activited T-cells (Dardenne et al. Immunology 33, 643 (1977), also show activity in the rosette inhibition assay. Thus a specific method for quantitation of FTS is needed to define the physiological and pathological characteristics of FTS, especially in vivo.
A radioimmunoassay (RIA) specific for FTS is one such useful technique for quantitation of this nonapeptide, however development of this technique has not been possible due to lack of antibodies specific for FTS and due to unavailability of suitable radioactive tracers. Recently, a monoclonal antibody against FTS (MA-FTS) using standard hybridoma technology was generated (Ogha et al., Clin. Exp. Immunol., 47, 725 (1982), U.S. patent application Ser. No. 305,579). This antibody specifically binds to synthetic FTS as well as to the substance in peripheral human blood that is detected in the rosette inhibition assay and is therefore useful in the assay of FTS. Radiolabeled analogues, however, have not been previously available because native porcine FTS lacks a phenolic ring through which a radiolabel can be attached.